Swing Hacks

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Chapter 3, Tables and Trees

#24 Create a JDBC Table Model

HACK

H A C K

#24

Create a JDBC Table Model

Hack #24

Bring your database tables into Swing with a minimum of hassle.

If you’ve worked with databases, you’ve probably also worked with the

tools they provide for quick table maintenance and queries: command-line

tools that are well suited to brief hack-and-slash work, but hard to work

with once you start dealing with any serious amount of data. It’s hard

enough to write the SQL command to return 10 or 20 columns in a query—

it’s even worse when the results word-wrap over the course of a dozen lines,

and you can’t tell where one result ends and another begins.

Wouldn’t it be nice to be able to throw the contents of any database table

into a Swing

JTable? Give it a few JDBC strings, toss it in a JFrame, and

pow!—instant GUI.

Building Connectivity

If you’ve worked with both JDBC and Swing, you’ll grasp the concept in one

sentence: use table metadata to build a Swing

TableModel from the database

table. If you haven’t, here’s the background you’ll need: JDBC provides an

abstract means of accessing databases. Java code to work with one database

should work with another, the only difference is in the way that JDBC

achieves a

Connection to the database, which is usually a matter of provid-

ing

Strings for:

• A driver class, which provides implementations of the various

java.sql

interfaces.

• A URL with which to connect to the database. This implies the use of

sockets, though that’s not necessarily the case. Some small embeddable

databases can live in the same JVM as your application.

• An optional username.

• An optional password.

Once you have the

Connection, you can begin to send commands (creation,

deletion, and altering of tables) or queries to the database by creating

Statements from the Connection. You can also use the Connection to get

metadata about the database, like what kinds of features it supports, how

long certain strings can be, etc. More importantly for this hack, it allows you

to discover what tables are in the database, what columns they have, and

what types of data are in those columns.

So, given just a

Connection and the name of a table in the database, you can

build a Java representation of its contents with two queries. The first query

gets column metadata for the table and builds up arrays of the column

Create a JDBC Table Model #24

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names and their types. These can be mapped reasonably well to Java classes,

at least for whatever types you intend to support. The second query gets all

the data from the table. For each row, it gets each column’s value. This is

put into a two-dimensional array, which represents the entire contents of the

table.

With these two queries done, you have everything you need to support the

abstract methods of

AbstractTableModel:

•

getRowCount( ) is the length of the contents array that you create.

•

getColumnCount( ) is 0 if you have no contents, or the length of the first

item in the contents array (which is itself an array because

contents is a

two-dimensional array).

•

getValueAt( ) is the value at contents[row][col].

AbstractTableModel has utterly trivial implementations of getColumnClass( )

and getColumnName( ), so the first always returns Object.class, the second

returns “A”, “B”, “C”, etc.; holding onto column metadata from the first

query allows you to provide more useful implementations of these methods,

too.

Example 3-12 shows how the

JDBCTableModel is implemented.

Example 3-12. Populating a Swing TableModel from a database connection

import javax.swing.*;

import javax.swing.table.*;

import java.sql.*;

import java.util.*;

/** an immutable table model built from getting

metadata about a table in a jdbc database

*/

public class JDBCTableModel extends AbstractTableModel {

Object[][] contents;

String[] columnNames;

Class[] columnClasses;

public JDBCTableModel (Connection conn,

String tableName)

throws SQLException {

super( );

getTableContents (conn, tableName);

}

protected void getTableContents (Connection conn,

String tableName)

throws SQLException {